In irradiation therapy it is necessary to check and document that the alignment of the field of irradiation on the area of the body to be treated is accurate and as planned. In irradiation in a stationary field with cobalt-60- and with linear- and circular accelerator sources, photographic exposures are also produced using the therapeutic irradiation exiting from the body of the patient. It is desirable hereby to extend the exposure time to the entire period of irradiation, in order to ensure precise documentation and to be able to recognize errors, caused for instance, by changes in the position of the patient during the irradiation. The quality of such verification radiographs becomes less satisfactory as the therapeutic irradiation becomes harder, however, because the contrast in the primary irradiation image is very low due to the diminishing weakening of the irradiation by the bones, and in addition, the unstructured scattered radiation from the body of the patient is also superimposed on the image. Even larger anatomical details such as, e.g., bronchia, can then no longer be recognized in these radiographs and they are unsuitable for documentation.
Numerous attempts have been made to obtain recordings with satisfactory recognizable detail, in spite of the existing difficulties. Thus, Jevbratt et al., Acta Radiologica 10, 433 (1971) investigated the suitability of various types of film for verification radiographs at 6 MeV and found that the best contrast is achieved on high-silver-content material test film at high density. Such films can only be processed automatically in special slow machines, which are not customarily used in hospitals. Practical recognition of detail is impaired because the radiographs, in addition to the image-forming points of high density, also often contain irregularly shaped clear fields, which are caused by the shadows of the shielding blocks used in the therapy and which dazzle the eye. This difficulty can be overcome by recopying, it is true, and the contrast can also be further increased, but the noise is also increased. According to Jevbratt, et al., "lith films" are also suitable, but are rejected by him because of the special processing necessary. Jevbratt, et al. also found that the image contrast on the material test film can be further improved if the film is laid between lead foils during exposure.
According to Droege et al., Medical Physics 6, 487 (1979), the essential function of these foils is to reduce the scattered radiation/primary radiation ratio. This effect is not influenced by the type of metal foil at photon energies greater than 4 MeV. For satisfactory results, however, foils with a weight per unit area of at least 3 g/cm.sup.2 are required. For a cassette of the usual 24.times.30 cm size, this means an additional weight of over 4 kg, a load that the radiological personnel cannot reasonably be expected to handle.
Meertens et al., Phys. Med. Biol. 30, 313 (1985) review the present state of the film-foil art for verification in megavolt irradiation therapy and come to the conclusion that further improvement is unlikely. They therefore suggest a new type of liquid-ionization detector for digital measurement of the radiograph. In another publication, Medical Physics 12, 111 (1985), digital processing of film radiographs to improve detail recognizability is also suggested by Meertens.
It is now the object of the present invention to give a recording system for verification and documentation in irradiation therapy at photon energies above 1 MeV, which is improved compared with the known systems with respect to detail recognizability and image contrast, which gives satisfactorily recognizable detail also at densities below 2.3, and whose recording material can be processed with the processing machines usually available in X-ray departments, and which after this processing has an immediate satisfactory image quality.
Further objects are evident from the following description.